Expression of eukaryotic membrane proteins in eukaryotic and prokaryotic hosts.

The production of membrane proteins of high purity and in satisfactory yields is crucial for biomedical research. Due to their involvement in various cellular processes, membrane proteins have increasingly become some of the most important drug targets in modern times. Therefore, their structural and functional characterization is a high priority. However, protein expression has always been more challenging for membrane proteins than for soluble proteins. In this review, we present four of the most commonly-used expression systems for eukaryotic membrane proteins. We describe the benefits and drawbacks of bacterial, yeast, insect and mammalian cells. In addition, we describe the different features (growth rate, yield, post-translational modifications) of each expression system, and how they are influenced by the construct design and modifications of the target gene. Cost-effective and fast-growing E. coli is mostly selected for the production of small, simple membrane proteins that, if possible, do not require post-translational modifications but has the potential for the production of bigger proteins as well. Yeast hosts are advantageous for larger and more complex proteins but for the most complex ones, insect or mammalian cells are used as they are the only hosts able to perform all the post-translational modifications found in human cells. A combination of rational construct design and host cell choice can dramatically improve membrane protein production processes.

Membrane Protein Production in Insect Cells.

Membrane proteins are an essential part of the machinery of life. They connect the interior and exterior of cells, play an important role in cell signaling and are responsible for the influx and efflux of nutrients and metabolites. For their structural and functional analysis high yields of correctly folded and modified protein are needed. Insect cells, such as Sf9 cells, have been one of the major expression hosts for eukaryotic membrane proteins in structural investigations during the last decade, as they are easier to handle than mammalian cells and provide more natural posttranslational modifications than microbial systems. Here we describe general techniques for establishing and maintaining insect cell cultures, the generation and amplification of recombinant baculovirus stocks using the flashBAC™ or Bac-to-Bac™ systems, membrane protein production, as well as the production of membrane preparations for extraction and purification experiments.

Detergent-Free Membrane Protein Purification Using SMA Polymer.

One of the big challenges for the study of structure and function of membrane proteins is the need to extract them from the membrane. Traditionally this was achieved using detergents which disrupt the membrane and form a micelle around the protein, but this can cause issues with protein function and/or stability. In 2009 an alternative approach was reported, using styrene maleic acid (SMA) copolymer to extract small discs of lipid bilayer encapsulated by the polymer and termed SMALPs (SMA lipid particles). Since then this approach has been shown to work for a range of different proteins from many different expression systems. It allows the extraction and purification of a target protein while maintaining a lipid bilayer environment. Recently this has led to several new high-resolution structures and novel insights to function. As with any method there are some limitations and issues to be aware of. Here we describe a standard protocol for preparation of the polymer and its use for membrane protein purification, and also include details of typical challenges that may be encountered and possible ways to address those.

Observation of arenavirus nucleoprotein heptamer assembly.

Arenaviruses are enveloped viruses containing a segmented, negative, and ambisense single-stranded RNA genome wrapped with a nucleoprotein (NP). The NP is the most abundant viral protein in infected cells and plays a critical role in both replication/transcription and virion assembly. The NP associates with RNA to form a ribonucleoprotein (RNP) complex, and this implies self-assembly while the exact structure of this polymer is not yet known. Here, we report a measurement of the full-length Mopeia virus NP by negative stain transmission electron microscopy. We observed RNP complex particles with diameter 15 ± 1 nm as well as symmetric circular heptamers of the same diameter, consistent with previous observations.

Membrane protein extraction and purification using partially-esterified SMA polymers.

Styrene maleic acid (SMA) polymers have proven to be very successful for the extraction of membrane proteins, forming SMA lipid particles (SMALPs), which maintain a lipid bilayer around the membrane protein. SMALP-encapsulated membrane proteins can be used for functional and structural studies. The SMALP approach allows retention of important protein-annular lipid interactions, exerts lateral pressure, and offers greater stability than traditional detergent solubilisation. However, SMA polymer does have some limitations, including a sensitivity to divalent cations and low pH, an absorbance spectrum that overlaps with many proteins, and possible restrictions on protein conformational change. Various modified polymers have been developed to try to overcome these challenges, but no clear solution has been found. A series of partially-esterified variants of SMA (SMA 2625, SMA 1440 and SMA 17352) has previously been shown to be highly effective for solubilisation of plant and cyanobacterial thylakoid membranes. It was hypothesised that the partial esterification of maleic acid groups would increase tolerance to divalent cations. Therefore, these partially-esterified polymers were tested for the solubilisation of lipids and membrane proteins, and their tolerance to magnesium ions. It was found that all partially esterified polymers were capable of solubilising and purifying a range of membrane proteins, but the yield of protein was lower with SMA 1440, and the degree of purity was lower for both SMA 1440 and SMA 17352. SMA 2625 performed comparably to SMA 2000. SMA 1440 also showed an increased sensitivity to divalent cations. Thus, it appears the interactions between SMA and divalent cations are more complex than proposed and require further investigation.

Brothers in Arms: Structure, Assembly and Function of Arenaviridae Nucleoprotein.

Arenaviridae is a family of viruses harbouring important emerging pathogens belonging to the Bunyavirales order. Like in other segmented negative strand RNA viruses, the nucleoprotein (NP) is a major actor of the viral life cycle being both (i) the necessary co-factor of the polymerase present in the L protein, and (ii) the last line of defence of the viral genome (vRNA) by physically hiding its presence in the cytoplasm. The NP is also one of the major players interfering with the immune system. Several structural studies of NP have shown that it features two domains: a globular RNA binding domain (NP-core) in its N-terminal and an exonuclease domain (ExoN) in its C-terminal. Further studies have observed that significant conformational changes are necessary for RNA encapsidation. In this review we revisited the most recent structural and functional data available on Arenaviridae NP, compared to other Bunyavirales nucleoproteins and explored the structural and functional implications. We review the variety of structural motif extensions involved in NP-NP binding mode. We also evaluate the major functional implications of NP interactome and the role of ExoN, thus making the NP a target of choice for future vaccine and antiviral therapy.